refactor: eliminate code duplication (DRY principle) (#211)

## Problem Solved
Previous refactoring added 105 lines (1476→1581) by extracting code into separate files without eliminating duplication. This refactor focuses on reducing code size through deduplication while maintaining file separation for maintainability.

## Summary
- Reduced total lines from 1581 to 1519 (-62 lines, 3.9% reduction)
- Eliminated duplicate patterns using generic helpers and factory functions
- Maintained file structure for maintainability
- Zero functional changes - same behavior

### Phase 1: DRY use-quota-manager.tsx
- Created parseStorageCount() helper (eliminates 6x localStorage read duplication)
- Created createQuotaChecker() factory (consolidates 3 check function bodies)
- Created createQuotaIncrementer() factory (consolidates 3 increment function bodies)
- Result: 242→247 lines (+5 lines, but fully DRY with eliminated duplication)

### Phase 2: DRY chat-panel.tsx (1176→1109 lines, -67 lines)

#### 2.1: Extract checkAllQuotaLimits helper
- Replaced 3 occurrences of 18-line quota check blocks
- Saved 36 lines

#### 2.2: Extract sendChatMessage helper
- Replaced 3 occurrences of 21-line sendMessage+headers blocks
- Saved 42 lines

#### 2.3: Extract processFilesAndAppendContent helper
- Replaced 2 occurrences of file processing loops
- Handles PDF, text, and image files uniformly
- Async helper with optional image parts parameter

lib/use-file-processor.tsx

@@ -0,0 +1,110 @@
+"use client"
+
+import { useState } from "react"
+import { toast } from "sonner"
+import {
+    extractPdfText,
+    extractTextFileContent,
+    isPdfFile,
+    isTextFile,
+    MAX_EXTRACTED_CHARS,
+} from "@/lib/pdf-utils"
+
+export interface FileData {
+    text: string
+    charCount: number
+    isExtracting: boolean
+}
+
+/**
+ * Hook for processing file uploads, especially PDFs and text files.
+ * Handles text extraction, character limit validation, and cleanup.
+ */
+export function useFileProcessor() {
+    const [files, setFiles] = useState<File[]>([])
+    const [pdfData, setPdfData] = useState<Map<File, FileData>>(new Map())
+
+    const handleFileChange = async (newFiles: File[]) => {
+        setFiles(newFiles)
+
+        // Extract text immediately for new PDF/text files
+        for (const file of newFiles) {
+            const needsExtraction =
+                (isPdfFile(file) || isTextFile(file)) && !pdfData.has(file)
+            if (needsExtraction) {
+                // Mark as extracting
+                setPdfData((prev) => {
+                    const next = new Map(prev)
+                    next.set(file, {
+                        text: "",
+                        charCount: 0,
+                        isExtracting: true,
+                    })
+                    return next
+                })
+
+                // Extract text asynchronously
+                try {
+                    let text: string
+                    if (isPdfFile(file)) {
+                        text = await extractPdfText(file)
+                    } else {
+                        text = await extractTextFileContent(file)
+                    }
+
+                    // Check character limit
+                    if (text.length > MAX_EXTRACTED_CHARS) {
+                        const limitK = MAX_EXTRACTED_CHARS / 1000
+                        toast.error(
+                            `${file.name}: Content exceeds ${limitK}k character limit (${(text.length / 1000).toFixed(1)}k chars)`,
+                        )
+                        setPdfData((prev) => {
+                            const next = new Map(prev)
+                            next.delete(file)
+                            return next
+                        })
+                        // Remove the file from the list
+                        setFiles((prev) => prev.filter((f) => f !== file))
+                        continue
+                    }
+
+                    setPdfData((prev) => {
+                        const next = new Map(prev)
+                        next.set(file, {
+                            text,
+                            charCount: text.length,
+                            isExtracting: false,
+                        })
+                        return next
+                    })
+                } catch (error) {
+                    console.error("Failed to extract text:", error)
+                    toast.error(`Failed to read file: ${file.name}`)
+                    setPdfData((prev) => {
+                        const next = new Map(prev)
+                        next.delete(file)
+                        return next
+                    })
+                }
+            }
+        }
+
+        // Clean up pdfData for removed files
+        setPdfData((prev) => {
+            const next = new Map(prev)
+            for (const key of prev.keys()) {
+                if (!newFiles.includes(key)) {
+                    next.delete(key)
+                }
+            }
+            return next
+        })
+    }
+
+    return {
+        files,
+        pdfData,
+        handleFileChange,
+        setFiles, // Export for external control (e.g., clearing files)
+    }
+}